Processes and apparatuses for thermal cleaning of offgases, wherein the offgases are supplied to a heated combustion chamber and oxidized to a flue gas, are known. If non-oxygen-containing offgases are used, the supply of an additional oxidizing agent (e.g. air) is required. Such apparatuses and processes are usually referred to as thermal offgas cleaning. They serve to clean the contaminated offgases.
This cleaning frequently comprises the oxidation of organic substances toward the non-toxic compounds carbon dioxide and water vapour in said flue gas if a recovery is impossible or undesirable.
The cleaning may, in the case of the haloorganosilicon compounds treated in this invention in the offgas, also comprise the oxidation of these substances/compounds toward the halogen-containing flue gases comprising HCl and Cl2, HBr and Br2, and HI and I2, as well as the aforementioned compounds carbon dioxide and water vapour. The gas formed after the oxidation is referred to as flue gas. The conversion products mentioned are thus present in the flue gas.
For economically viable operation of such an apparatus, in the vast majority of cases, further utilization of the reaction enthalpy typically released by the oxidation is provided, which is present in the form of tangible energy of the hot flue gas after the oxidation. One possible use thereof is for preheating of the offgases to be treated in order to reduce the fuel requirement of the overall process.
Extremely high offgas preheating temperatures and hence low fuel consumptions can be achieved by what is called regenerative offgas preheating based on cyclically switchable storage beds.
This involves heating the offgas to be cleaned in a hot regenerator, in the course of which the latter is cooled. The heated offgas subsequently enters an oxidation zone in which constituents of the offgas are oxidized at temperatures in the range from typically 800° C. to 1200° C. The hot flue gas thus obtained is subsequently by means of a cold regenerator, as a result of which the latter is heated and the flue gas is cooled. This process is periodically reversed by switching the flow direction, such that only the temperature profiles shift in the regenerators.
This is referred to as an RTO process (regenerative thermal oxidation) and an RTO plant (cf., for example, VDI-Richtlinie [Association of German Engineers guideline] 2442: “Abgasreinigung—Verfahren and Technik der thermischen Abgasreinigung” [Waste gas cleaning—Methods of thermal waste gas cleaning], updated: March 2006).
Numerous processes, however, give rise to offgases which comprise dust-forming constituents inter alia, for example organosilicon compounds. In this case, use of the RTO plants and processes has to date been opposed by the fact that the plant becomes covered and hence blocked by the oxidation products in the flue gas (predominantly SiO2) formed from the organosilicon compounds in the offgas.
The offgases comprising haloorganosilicon compounds treated according to the present invention typically result, in the course of oxidation, in the formation of highly corrosive conversion products in the flue gas.
Such conversion products are, in combination with the oxidation product water or water vapour (see above), especially HCl when the halogen is chlorine and this is present in the flue gas. The other conversion products in the flue gas comprising the halogen from the haloorganosilicon compounds, such as HBr and HI, also form extremely corrosive acids (e.g. hydrochloric acid) in the flue gas together with the water vapour produced.
This is especially true when, during the process, the temperature in the apparatus goes below the dew point of the components in the flue gas as a result of cooling over the heat storage material.
The aforementioned dew point is referred to as the acid dew point since it corresponds to the dew point of the acid relevant in each case (for example dilute hydrochloric acid from HCl) and is typically much higher than the dew point of water without the presence of acid-forming components. The result of this is that, in the course of cooling of said flue gases, condensation occurs significantly earlier than would be expected without the acid-forming components.
If the haloorganosilicon compounds in the offgas also comprise sulphur atoms as well as the aforementioned halogens, there is additionally also formation of conversion products in the form of SO2 and/or SO3 in the flue gas, which, in conjunction with water, form the likewise extremely corrosive compounds H2SO3 (sulphurous acid) and H2SO4 (sulphuric acid) when the temperature goes below the acid dew point of a flue gas comprising such compounds.
The aforementioned conversion products in the flue gas, in combination with the water vapour, mean that essential apparatuses in such a plant must consist of a material which can withstand both the temperatures therein and the corrosion. Especially in the case of prolonged operation of the process, this, however, is impossible under economic conditions.
The offgas treated is thus a substance mixture which is not a product of value as such and therefore cannot justify great investment in the apparatus for treatment. Therefore, the apparatus has to be cleaned or replaced at regular intervals if the aforementioned blockage and corrosion have reached a no longer acceptable degree.
In addition, it should be ensured that—in order to avoid corrosion—the temperature does not go below the acid dew point of the flue gas in the plant in order to at least minimize corrosion and thus to prolong operating time and increase availability. This, however, requires higher preheating temperatures for the offgas and hence high additional operating costs to achieve these high preheating temperatures.
A significant constituent of the aforementioned RTO plants is a heat exchanger as an apparatus for recovery of the aforementioned reaction enthalpy from the oxidized flue gas stream. The heat exchanger in such processes/plants is configured as a regenerator.
A regenerator is a heat exchanger in which input of (thermal) energy and output of (thermal) energy are decoupled from one another in terms of time.
In order to achieve such decoupling in terms of time, the (thermal) energy in such regenerators is stored in a heat storage material, which is typically a solid with a high specific heat storage capacity (cp, expressed in J/(kg·K)), by contacting a gas (the flue gas here) at high temperature with this heat storage material, as a result of which the gas is cooled and the heat storage material is correspondingly heated.
In a second step, a gas to be preheated (the offgas here) is then contacted with the heated heat storage material, as a result of which the heat storage material is in turn cooled and the desired preheating of the gas is achieved.
If the heat storage material used does not have sufficiently high specific heat storage capacity, this can be compensated for by a correspondingly increased mass of heat storage material, such that a sufficiently high heat storage capacity is present overall. The heat is transferred from the flue gas to the heat storage material and back to the offgas via the surface of the heat storage material, and so heat storage shapes having a high surface-to-volume ratio are typically used. Such shapes are, for example, monoliths (e.g. honeycombs) or beds of various kinds (e.g. saddles, Berl saddles or balls).
For regenerators in RTO plants for the treatment of organosilicon compounds, ball-shaped random packings in particular are advantageous to a certain degree because they can also be executed in embodiments in which the heat storage material is in the form of beds.
Such beds have the advantage that they can be moved within the regenerator. This facilitates the exchange of the heat storage material.
The amount of heat storage material correlates directly with the necessary volume of the regenerator, which in turn correlates directly with the capital costs. This is not the only reason why such regenerators with beds in RTO plants which, as explained above, do not treat products of value are highly economically advantageous.
Furthermore, the heat storage material, in the specific case of plants which treat the offgases comprising haloorganosilicon compounds relevant in the present invention, due to the aforementioned baked-on material resulting from SiO2 from the flue gas, has to be freed of this baked-on material at regular intervals in order that the regenerator still fulfils the desired function.
This recurrent maintenance, in the case of regenerators with heat storage materials which are not beds (for instance those in the form of honeycombs), causes the deinstallation of the heat storage material from the regenerator, the cleaning thereof, and then reinstallation.
If it is possible to make the time delay between the heating operation of the heat storage material and the cooling operation of the heat storage material sufficiently long, i.e. to keep a certain period free between the cooling and later heating, this maintenance can be accomplished during this period.
Typically, however, this is impossible and the cost and inconvenience is in many cases unacceptable, both in terms of the working conditions and in terms of labour. In this context, many manufacturers of such plants, in their technical tender documents, have to date excluded the treatment of offgases comprising organosilicon compounds.
A regenerator with such a bed for use in an RTO plant and a process for operation thereof in connection with offgases comprising organosilicon compounds, which form the aforementioned deposits from the flue gas, is described in DE 103 57 696.
DE 103 57 696 discloses, more particularly, a process for thermal cleaning of offgases comprising aforementioned organosilicon compounds, in which a regenerator with a bed is used, the bed being withdrawn periodically from the regenerator for the purpose of cleaning to remove the deposits.
As described in DE 103 57 696, the cleaning to remove the deposits is not effected during the normal operation of the RTO plant but during a break in operation in which cleaning of the offgases is impossible and hence the availability of offgas cleaning is restricted. Nevertheless, the withdrawal of beds is several times easier than the deinstallation of any internals in the regenerator, for example monolithic heat storage materials.
The offgases treated in DE 103 57 696, however, do not include haloorganosilicon compounds.
One disadvantage associated with the aforementioned beds is that the bed as such has an abrasive effect on the apparatus in which it is present, since the bed, in the course of withdrawal from and charging of the regenerator, galls on the inner surface of the regenerator and can thus lead to material removal at the inner surface thereof
In combination with the aforementioned haloorganosilicon compounds in the offgas which lead to corrosive conversion products in the flue gas, it is exactly this abrasive action that is particularly problematic since this activates the surface of the apparatus and thus promotes corrosion. This is especially true when the temperature goes below the acid dew point of the flue gas as a result of cooling thereof in the course of the process, as a result of which the aforementioned particularly corrosive conversion products are formed, which attack and damage the apparatus.
DE 103 57 696 thus solves the problem of the economically simpler cleaning of the heat storage material, but leads to further problems in conjunction with the treatment of offgases comprising haloorganosilicon compounds.
These problems relating especially to the possibility of industrial exploitation of the heat content of the hot flue gas comprising water vapour and conversion products of the haloorganosilicon compounds.
The apparatus known from DE 103 57 696 and the process described therein are unable to treat such haloorganosilicon compounds in a viable manner since either the outlet temperature of the flue gases from the apparatus has to be set so high that the temperature reliably does not go below the acid dew point, which prevents complete utilization of the energy content of the hot flue gas, or since, on the other hand, the apparatus, when the temperature goes below the acid dew point, cannot be operated to an economically acceptable degree as a result of the occurrence of corrosion.
Proceeding from the above-described problem of the prior art, it is therefore an object of the present invention to provide an apparatus and a process which retains the advantages according to DE 103 57 696 with regard to the more economic maintenance of the regenerator, but at the same time enable treatment also of offgases comprising haloorganosilicon compounds.